Seminar by Prof. Dr. hab Le Van Hoang, Dr. Dao Phuc Lam, and Dr. Ho Si Lanh

On June 9, 2023, the lecture from Prof. Dr. Le Van Hoang, Dr. Dao Phuc Lam, and Dr. Ho Si Lanh talk place in meeting room B.

Prof. Dr. Le Van Hoang presents a talk entitled “Thermal-magnetic effect on magnetoexciton energy spectra in monolayer transition metal dichalcogenides”

Abstract

It is widely comprehended that temperature may cause phonon-exciton scattering, enhancing the energy level’s linewidth and leading to some spectrum shifts. However, in the present paper, we suggest a different mechanism that allows the thermal motion of the exciton’s center of mass (c.m.) to affect the magnetoexciton energies in monolayer dichalcogenides (TMDCs). By the nontrivial but precise separation of the c.m. motion from an exciton in a monolayer TMDC with a magnetic field, we obtain an equation for the relative motion containing a motional Stark term proportional to the c.m. pseudomomentum, related to the temperature of the exciton gas but neglected in the previous studies. Solving the Schrödinger equation without omitting the motional Stark potential at room temperature shows approximately a few meV thermal-magnetic shifts in the exciton energies, significant enough for experimental detection. Moreover, this thermal effect causes a change in exciton radius and diamagnetic coefficient and enhances the exciton lifetime as a consequence. Surprisingly, the thermo-induced motional Stark potential breaks the system’s SO(2) symmetry, conducting new peaks in the exciton absorption spectra at room temperature besides those of the s states. This mechanism could be extended for other magneto quasiparticles such as trions and biexcitons.

Dr. Dao Phuc Lam presents a talk entitled "Stabilization of lateritic soils using Fly ash/ GGBS based geopolymer”

Abstract

Lateritic soils are weathered soil, which is available around the tropical regions in the world. However, the application of this soil as a construction material is limited due to the complexity of its properties. In general, for the improvement of the mechanical properties of soil, cement is used as a common method. However, the use of cement results in the emission of carbon dioxide. Recently, geopolymer is a green binder, which is successfully applied in concrete and mortar. For the stabilization of laterite soil, few studies used geopolymer to treat this soil. The results from some previous studies indicated that geopolymer can be used as a new green binder to stabilize laterite soil for sub-base and sub-grade of pavement, in which the compressive strengths at both early and later ages satisfied the strength requirement by some countries. This implies that geopolymer can be applied as a potential binder to reinforce laterite soil for sustainable development.

Dr. Ho Si Lanh presents a talk entitled “Analysis of strength development in cement-treated soils under different curing conditions through micro-structural and chemical investigations”

Abstract

This study investigated the strength development of cement-treated soils by considering not only the interactions among cement hydration and pozzolanic reaction, but also the carbonation process. Indeed, the chemical and microstructural changes in cement-treated soils are due to carbonation combined with previous reactions and no studies took into account all processes at the same time. Three soil specimen types (specifically, sand, sand-loam, and sand-bentonite mixtures) were cured under sealed and drying conditions and their unconfined compressive strength, microstructural and chemical properties subsequently measured over time. Under drying conditions, behaviors varied with soil mineralogy. The compressive strength of all mixtures significantly increased with decreasing total micropore induced by carbonation and suction effects. However, in the sand-clay mixture under long-term drying, the compressive strength remained constant or slightly decreased after 28 days, correlating with the increase in large capillary pore caused by shrinkage of bentonite and C–S–H carbonation. These results revealed that carbonation could have both positive and negative impacts on strength development of cement-treated soils.